626 THE CIRCULATION IN ASPHYXIA. [BOOK n. 



easily through the capillaries than does blood less venous in 

 character. 



This increased peripheral resistance and the high blood-pressure 

 to which it gives rise, while tending to increase the distension 

 of the left ventricle and so indirectly helping to augment the 

 force of the heart's beat, soon becomes a direct obstacle to the heart 

 emptying itself of its contents. On the other hand, the laboured 

 respiratory movements favour the flow of venous blood towards 

 the heart, which in consequence becomes more and more full. 

 This repletion is moreover assisted by the marked infrequency 

 of the beats which is soon developed. This in turn depends in 

 part on the cardio-inhibitory centre in the medulla being stimu- 

 lated by the venous blood; but, as we have previously seen, 

 cannot be wholly accounted for in this way. The increased 

 resistance in front, the augmented supply from behind, and the 

 long pauses between the strokes, all concur in distending the 

 heart more and more. 



When the large veins have become full of blood the inspiratory 

 movements can no longer have their usual effect in facilitating 

 the venous flow into the right auricle. The chief effect of the 

 chest movement, as far as the circulation is concerned, is to 

 widen and so to increase the capacity of the pulmonary vessels, and 

 at the same time to diminish the pressure around the large 

 arteries ; hence the marked sinking of the blood-pressure during 

 each inspiratory movement. 



The distension of the cardiac cavities, at first favourable to the 

 heart-beat, as it increases becomes injurious ; and the cardiac 

 tissues after a while become enfeebled by the action of the 

 venous blood, so that the strokes of the heart become weaker and 

 irregular. 



On account of this increasing feebleness of the heart's beat, 

 accompanied by more or less irregularity, the blood-pressure, in 

 spite of the continued arterial constriction, begins to fall, since 

 less and less blood is pumped into the arterial system ; the 

 boldness of the pulse-curves at this stage is chiefly due to 

 the infrequency of the strokes. As the quantity which passes 

 from the heart into the arteries becomes less second by second, 

 the pressure gets lower and lower, the descent being assisted by 

 the exhaustion of the vaso-motor centre, until almost before the 

 last beats it has sunk to zero. Thus at the close of asphyxia, 

 while the heart and venous system are distended with blood, the 

 arterial system is less than normally full. 



389. While changes occurring primarily in the respiratory 

 system thus affect the vascular system, conversely changes occurring 

 primarily in the vascular system affect the respiratory system. 

 Two kinds of change in the vascular system bearing on two parts 

 of the respiratory system deserve especial attention. 



In the first place the respiratory mechanism may be affected 



